Reducing crimping damage to a polymer scaffold

ABSTRACT

A medical device includes a polymer scaffold crimped to a catheter having an expansion balloon. The scaffold is crimped to the catheter by a multi-step process for increasing scaffold-catheter yield following a crimping sequence. Damage reduction during a crimping sequence includes modifying blades of a crimper, adopting a multi-step crimping sequence, and inflating a supporting balloon to support the scaffold during crimping.

This is a divisional of U.S. application Ser. No. 12/861,719 filed Aug.23, 2010, the contents of which are hereby incorporated by reference inits entirety for all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to drug-eluting medical devices; moreparticularly, this invention relates to processes for crimping apolymeric scaffold to a delivery balloon.

2. Background of the Invention

Referring to FIG. 1A, there is shown a perspective view of a crimpingassembly 20 that includes three rolls 123, 124, 125 used to position aclean sheet of non-stick material between the crimping blades and ametal stent prior to crimping. For example, upper roll 125 holds thesheet secured to a backing sheet. The sheet is drawn from the backingsheet by a rotating mechanism (not shown) within the crimper head 20. Asecond sheet is dispensed from the mid roll 124. After crimping, thefirst and second (used) sheets are collected by the lower roll 123. Asan alternative to rollers dispensing a non-stick sheet, each metal stentmay be covered in a thin, compliant protective sheath before crimping.

FIG. 1B illustrates the positioning the first sheet 125 a and secondsheet 124 a relative to the wedges 22 and a metal stent 100 within theaperture of the crimping assembly 20. As illustrated each of the twosheets are passed between two blades 22 on opposite sides of the stent100 and a tension T1 and T2 applied to gather up excess sheet materialas the iris of the crimping assembly is reduced in size via theconverging blades 22.

The dispensed sheets of non-stick material (or protective sheath) areused to avoid buildup of coating material on the crimper blades forstents coated with a therapeutic agent. The sheets 125 a, 124 a arereplaced by a new sheet after each crimping sequence. By advancing aclean sheet after each crimp, accumulation of contaminating coatingmaterial from previously crimped stents is avoided. By using replaceablesheets, stents having different drug coatings can be crimped using thesame crimping assembly without risk of contamination or buildup ofcoating material from prior stent crimping.

The art recognizes a variety of factors that affect a polymericscaffold's ability to retain its structural integrity when subjected toexternal loadings, such as crimping and balloon expansion forces. Theseinteractions are complex and the mechanisms of action not fullyunderstand. According to the art, characteristics differentiating apolymeric, bio-absorbable scaffold of the type expanded to a deployedstate by plastic deformation from a similarly functioning metal scaffoldare many and significant. Indeed, several of the accepted analytic orempirical methods/models used to predict the behavior of metallicscaffolds tend to be unreliable, if not inappropriate, as methods/modelsfor reliably and consiscaffoldly predicting the highly non-linearbehavior of a polymeric load-bearing structure of a balloon-expandablescaffold. The models are not generally capable of providing anacceptable degree of certainty required for purposes of implanting thescaffold within a body, or predicting/anticipating the empirical data.

Moreover, it is recognized that the state of the art in medicaldevice-related balloon fabrication, e.g., non-compliant balloons forscaffold deployment and/or angioplasty, provide only limited informationabout how a polymeric material might behave when used to support a lumenwithin a living being via plastic deformation of a network of ringsinterconnected by struts. In short, methods devised to improvemechanical features of an inflated, thin-walled balloon structure, mostanalogous to mechanical properties of a pre-loaded membrane when theballoon is inflated and supporting a lumen, simply provides little, ifany insight into the behavior of a deployed polymeric scaffold. Onedifference, for example, is the propensity for fracture or cracks todevelop in a polymer scaffold. The art recognizes the mechanical problemas too different to provide helpful insights, therefore, despite ashared similarity in class of material. At best, the balloon fabricationart provides only general guidance for one seeking to improvecharacteristics of a balloon-expanded, bio-absorbable polymericscaffold.

Polymer material considered for use as a polymeric scaffold, e.g. PLLAor PLGA, may be described, through comparison with a metallic materialused to form a stent, in some of the following ways. A suitable polymerhas a low strength to weight ratio, which means more material is neededto provide an equivalent mechanical property to that of a metal.Therefore, struts must be made thicker and wider to have the strengthneeded. The scaffold also tends to be brittle or have limited fracturetoughness. The anisotropic and rate-dependant inelastic properties(i.e., strength/stiffness of the material varies depending upon the rateat which the material is deformed) inherent in the material onlycompound this complexity in working with a polymer, particularly,bio-absorbable polymer such as PLLA or PLGA.

Processing steps performed on, and design changes made to a metal stentthat have not typically raised concerns for, or required carefulattention to unanticipated changes in the average mechanical propertiesof the material, therefore, may not also apply to a polymer scaffold dueto the non-linear and sometimes unpredictable nature of the mechanicalproperties of the polymer under a similar loading condition. It issometimes the case that one needs to undertake extensive validationbefore it even becomes possible to predict more generally whether aparticular condition is due to one factor or another—e.g., was a defectthe result of one or more steps of a fabrication process, or one or moresteps in a process that takes place after scaffold fabrication, e.g.,crimping? As a consequence, a change to a fabrication process,post-fabrication process or even relatively minor changes to a scaffoldpattern design must, generally speaking, be investigated more thoroughlythan if a metallic material were used instead of the polymer. Itfollows, therefore, that when choosing among different polymericscaffold designs for improvement thereof, there are far less inferences,theories, or systematic methods of discovery available, as a tool forsteering one clear of unproductive paths, and towards more productivepaths for improvement, than when making changes in a metal stent.

It is recognized, therefore, that, whereas inferences previouslyaccepted in the art for stent validation or feasibility when anisotropic and ductile metallic material was used, such inferences wouldbe inappropriate for a polymeric scaffold. A change in a polymericscaffold pattern may affect not only the stiffness or lumen coverage ofthe scaffold in its deployed state supporting a lumen, but also thepropensity for fractures to develop when the scaffold is crimped orbeing deployed. This means that, in comparison to a metallic stent,there is generally no assumption that can be made as to whether achanged scaffold pattern may not produce an adverse outcome, or requirea significant change in a processing step (e.g., tube forming, lasercutting, crimping, etc.). Simply put, the highly favorable, inherentproperties of a metal (generally invariant stress/strain properties withrespect to the rate of deformation or the direction of loading, and thematerial's ductile nature), which simplify the stent fabricationprocess, allow for inferences to be more easily drawn between a changedstent pattern and/or a processing step and the ability for the stent tobe reliably manufactured with the new pattern and without defects whenimplanted within a living being.

A change in the pattern of the struts and rings of a polymeric scaffoldthat is plastically deformed, both when crimped to, and when laterdeployed by a balloon, unfortunately, is not as easy to predict as ametal stent. Indeed, it is recognized that unexpected problems may arisein polymer scaffold fabrication steps as a result of a changed patternthat would not have necessitated any changes if the pattern was insteadformed from a metal tube. In contrast to changes in a metallic stentpattern, a change in polymer scaffold pattern may necessitate othermodifications in fabrication steps or post-fabrication processing, suchas crimping and sterilization.

One problem encountered with a polymer scaffold is the susceptibility todamage when being crimped to a balloon. Non-uniform forces appliedduring a crimping process can cause irregular deformations in struts ofa polymer scaffold, which can induce crack formation, and loss ofstrength. There is a continuing need to improve upon the crimpingmethods, or pre-crimping procedures used for polymer scaffold to reduceinstance of crack formation or irregular strut deformation duringscaffold production.

SUMMARY OF THE INVENTION

The invention provides a process and apparatus for crimping a polymerscaffold to a balloon. The polymer scaffold is expanded for placementwithin a lumen of the body by plastic deformation of the polymerscaffold. The crimping process used to place the scaffold on the balloonincludes, in one embodiment, an initial diameter reduction followed byfinal crimp steps. In one respect, the invention provides both amodified crimping apparatus and a modified process for crimping apolymer scaffold to reduce damage and/or improve batch yield for polymerscaffold-catheter assemblies following the crimping phase of aproduction process. Modifications include modifications to a crimpingblade, interior supports for a scaffold and the sequence of stepsincluded in crimping to achieve the desired results. All of theseimprovements used together, or only some have been found to improveresults significantly.

The invention addresses the problem of damage caused when a balloonexpandable polymer scaffold is crimped to a deployment balloon. When apolymer scaffold is placed in a crimping device for metallic stents,there is frequently occurring damage done to the scaffold structure bythe forces acting on the surfaces of the scaffold through crimper bladesas the scaffold is crimped to the balloon. A crimping device used formetallic stents uses metallic blades and in some cases blades withhardened tips. The devices are constructed in this way to allow frequentcrimping of metallic structure without pitting or deformation of theblades when deforming struts of metallic stents. Moreover, due to therelative hardness between the blades and metal stent struts, there isusually no significant structural damage to the strut of the metalstent. To the extent a blade tip bears into and forms an indentation ina metal stent strut, the stent may still perform in an acceptable mannerdue to the resilience and ductile properties of a metal. The polymerscaffold is much softer than a metal. As such, when a metal surface of ablade bears down on surfaces of the scaffold there is likelihood thatthe metal edge of the blade will permanently deform the strut by theformation of indentations, cuts or gouges in the polymer material.Unlike a metal strut, indentations raise concerns over crackpropagation, especially for a brittle polymer like PLLA. Indentationsare believed to occur mostly towards the end of the crimping sequencewhen the tips of the crimper blades are orientated more directly towardsthe scaffold surface.

One solution that has been proposed in the past is to enclose a drugeluting stent within a cylindrical sheath during a crimping process.However, when the diameter reduction is beyond a certain amount, acylindrical sheath covering cannot be used. A high degree of diameterreduction (beyond the compression range for the sheath covering) ofabout 2.5 to 3.0 times a starting diameter causes a sheath to buckle orfold over itself during the later stages of the diameter reduction. Thisbehavior by the sheath covering would cause more troubles than it wasintended to solve.

Polymer scaffolds are also more susceptible to irregular crimping forcesthat result in bent, twisted, or overlapping struts. These crimpingproblems are due to misalignment of the scaffold within the crimper.Static charge buildup on polymer surfaces are one cause for themisalignment, e.g., as when a polymer sheet slides over a polymerscaffold surface during crimping. However, it is also believed thatmisalignments that would normally be tolerated when crimping a metalstent can create irregular crimping of a polymer scaffold due to theproximity of struts in a polymeric scaffold. Closely-spaced struts cancause overlapping, twisting or bending of struts as struts abut oneanother during a diameter reduction within the crimper. This is usuallynot a concern for metal stents since the struts can be made thinner,allowing for more space between struts. Also, wider spaces can be formedfor metal stents since the starting diameter, before crimping, is closerto the crimped diameter. Improper crimping was also found when using acrimper that disposes polymer sheets between a scaffold and crimperblades. The polymer sheets move relative to each other as the irisreduces in size. When coming into contact with the scaffold, therefore,the sheets can induce twisting or misalignment of the scaffold withinthe crimper, which can lead to irregular crimping of the scaffold.

These and related problems are addressed by the invention. In oneembodiment there is a method for crimping a polymer scaffold to aballoon including providing a crimping assembly for crimping thescaffold from a first diameter to a reduced second diameter, thecrimping assembly including a plurality of movable blades, each bladehaving a hardness, a first side and a second side converging to form atip, the tips being arranged to collectively form an iris about arotational axis thereof, the iris defining a crimp aperture about whichthe movable blades are disposed; disposing a polymer material betweenedges of the blade tips and the scaffold surface to reduce the hardnessin the edges; supporting the scaffold including inflating a balloonwithin the scaffold to provide interior support to the scaffold, wherebyadjacent struts of the scaffold twisted irregularly by a crimper bladeare supported by the balloon surface to deter one of the struts fromoverlapping or twisting irregularly relative to the other strut; anddisplacing the plurality of movable wedges from the first diameter tothe reduced second diameter to reduce the diameter of the scaffold fromthe first scaffold diameter to a second scaffold diameter, respectively.

In another embodiment there is a method for crimping a polymer scaffoldto a balloon including providing a crimping assembly; providing apolymer coating on blade tips to soften a leading edge of the bladetips; supporting the scaffold during crimping by an inflated balloonwithin the scaffold wherein the balloon applies a radially outwardpressure to provide a stabilizing pressure to a strut displacing out ofplane or twisting due to uneven crimping forces applied to the strut ornear the strut; and displacing the plurality of movable blades from thefirst diameter to the reduced second diameter; wherein the balloonpressure is adjusted as the scaffold diameter is reduced by the tipincluding the leading edge such that the radially directed outward forceapplied on the scaffold by the balloon supports the scaffold to avoid orcompensate for irregular bending or twisting of scaffold structure.

In another embodiment there is a method for crimping a polymer scaffoldto a balloon including providing a crimping assembly for crimping thescaffold from a first diameter to a reduced second diameter, thecrimping assembly including the plurality of movable blades, a firstsheet of polymer film extending between a first and second pair ofopposed blades such that a portion of the first sheet extends across anaperture formed by the iris, and a second sheet of polymer filmextending between the first and second pair of opposed blades such thata portion of the second sheet also extends across the aperture; placingthe scaffold on a balloon; disposing the scaffold and balloon in theaperture such that the scaffold and balloon are located between thefirst and second sheets; inflating the balloon; and displacing theplurality of movable blades from about the first diameter to about thereduced second diameter to reduce the diameter; wherein the balloonpressure is adjusted as the scaffold diameter is reduced by the tip suchthat the radially directed outward force applied on the scaffold by theballoon supports the scaffold to avoid or compensate for irregularbending or twisting of scaffold structure caused by the first and secondsheets.

In another embodiment there is a method for crimping a polymer scaffoldto a balloon including placing a scaffold having a first diameter on asupport balloon; inflating the support balloon to a pressure forsupporting and stabilizing the scaffold at the first diameter while thescaffold is being crimped; crimping the scaffold from the first diameterto a second diameter while the first balloon is supported by the supportballoon; replacing the support balloon with a balloon catheter after thescaffold is reduced to the second diameter; and crimping the scaffold toa third diameter, less than the second diameter while the scaffold issupported on the balloon catheter.

In another embodiment there is an assembly for crimping a polymerscaffold to a balloon, the assembly including a plurality of movableblades, each blade having a hardness, a first side and a second sideconverging to form an edge, the edges arranged to collectively form aniris about a rotational axis thereof, the iris defining a crimp apertureabout which the movable blades are disposed; wherein the blade edge isone or both of a polymer coated to reduce the hardness of the blade, orformed as a blunted edge wherein the blunted edge is non-symmetricallydisposed about a bisecting line defining a line of action of the bladewhen the crimping mechanism adjusts the iris from a first to a seconddiameter.

In another embodiment there is a method for crimping a polymer scaffoldto a balloon including placing a scaffold having a first diameter on asupport balloon; the support balloon is pressurized to manipulate theorientation of the scaffold at first diameter. This orientation is usedto position the metal marker beads relative to the crimp head forspecific placement, with the use of a proximity sensor, e.g. a lasersensor, which is disposed within the crimp head. Lateral misplacement ofthe scaffold could lead to disparity between proximal and distal ends ofthe crimp blades. To ensure more uniformity of lengthwise crimping loadson the scaffold, thereby ensuring more uniformity in the crimped shape(particularly for longer scaffolds, e.g., 120 mm in length), thescaffold is aligned precisely within the crimper head using thealignment system.

As a further aspect of the processes described above, a scaffold may beremoved from the crimp head and rotated to a selected angular positionafter an initial crimp, or a subsequent reduced diameter beforereplacing a support balloon with the catheter delivery balloon. Byre-orienting the scaffold in this manner, one can establish moreuniformity in the crimp profile as the scaffold diameter is reduced. Forexample, when using polymer sheets as described in the process inconnection with FIG. 1B, the motion of the sheets relative to the bladesmay produce a non-uniform crimp of the scaffold about its circumferencedue to twisting or pulling in torsion the scaffold by the sheets(particularly when a static charge is present). Rotation of the scaffoldthrough an angle, e.g., 90 degrees, after a crimp may help to reduceproblems caused by polymer sheets.

The scope of the methods and apparatus of the invention furtherencompass processes that crimp the scaffolds described in US Pub. No.2010/0004735 and US Pub. No. 2008/0275537, and the scaffolds describedin U.S. application Ser. No. 12/561,971.

INCORPORATION BY REFERENCE

All publications and patent applications mentioned in this specificationare herein incorporated by reference to the same extent as if eachindividual publication or patent application was specifically andindividually indicated to be incorporated by reference, and as if eachsaid individual publication or patent application was fully set forth,including any figures, herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a prior art crimping assembly utilizing opposed filmsheets to provide a barrier between a drug coated stent and blades of aniris-type crimping assembly.

FIG. 1B is a cross-sectional view taken along a crimping axis of theiris-type crimping assembly of FIG. 1A. This drawing illustrates the useof tensioned polymeric sheets used to maintain a clean surface on thecrimper blades when metal stents carrying different drug-polymercoatings are crimped to a balloon.

FIG. 2 shows a first cross-sectional view of blades of an iris-typecrimper taken along the crimper axis when reducing a polymer scaffolddiameter from a first, large diameter to a second, smaller diameter.

FIG. 3 shows a second cross-sectional view of blades of the iris-typecrimper of FIG. 2 taken along the crimper axis when reducing the polymerscaffold diameter from a second diameter to a final crimped diameter.

FIGS. 4A and 4B show close-up views of an individual blade of thecrimper blades of FIGS. 2 and 3, respectively, as it contacts a surfaceof a polymer scaffold. In FIG. 4B the blade edge is bearing into thescaffold surface and causing an indentation to form. The blade may alsofurther deform an already bent or twisted strut. Furthermore, the bladetip can also cut through the polymer material.

FIG. 5 is a first disclosure of a pattern for a polymer scaffold crimpedto a balloon according to the invention.

FIG. 6 is a second disclosure of a pattern for a polymer scaffoldcrimped to a balloon according to the invention.

FIGS. 7A and 7B show a first and second disclosure of a polymer coatingfor a crimper blade. The coatings have the effect of making the bladeedge softer so as to reduce damaging indentations and cutting or gouginginto the scaffold when the scaffold is brought into contact with arelatively hard and sharp crimper blade edge.

FIG. 7C shows a replaceable polymer insert for a blade for reducingdamage to a polymer scaffold.

FIGS. 8A-8B shows modified crimper blade edges intended for reducingdamage to a polymer scaffold.

FIG. 9 shows a perspective view of a crimping device having a pluralityof movable blades that form an iris for crimping.

FIG. 10 is a third disclosure of a pattern for a polymer scaffoldcrimped to a balloon according to the invention

FIGS. 11A-11B show pinching and bending of strut sections for a polymerscaffold having an as-deployed strut pattern as depicted in FIG. 5 whena conventional crimping process is used to crimp the scaffold to aballoon.

FIGS. 12A-12B show cuts, indentations and gouges formed in the polymermaterial for a polymer scaffold having an as-deployed strut pattern asdepicted in FIG. 5 when a conventional crimping process is used to crimpthe scaffold to a balloon.

FIG. 13 shows a crimped polymer scaffold having an as-deployed strutpattern as depicted in FIG. 5 when processes according to the inventionare used.

FIG. 14A-14C show irregularities and damages to a polymer scaffoldhaving an as-deployed strut pattern as depicted in FIG. 10 when aconventional crimping process is used to crimp the scaffold to aballoon.

FIG. 15 shows a crimped polymer scaffold having an as-deployed strutpattern as depicted in FIG. 10 when processes according to the inventionare used.

DETAILED DESCRIPTION OF EMBODIMENTS

As discussed earlier, the invention arose out of a need to solve aproblem of high rejection rates for balloon expandable polymer scaffoldscrimped to a deployment balloon. Polymer scaffolds were being rejectedbecause the structure was being irregularly deformed by the crimper,e.g., struts overlapping each other or being twisted into abnormalshapes, and because there were a high number of cracks and/orindentations formed in the scaffold. Subsequent balloon deployment,followed by accelerated life testing, cyclic and static load testing ofthe scaffold in its deployed state revealed that the aforementioneddamage done to the scaffold was unacceptable. This damage to thescaffold when crimped resulted in a relatively high probability offailure as one or more struts fractured when the scaffold is loaded by avessel, or the scaffold expanded improperly, thereby not properlysupporting a vessel. The causes for this damage, while generally knownwere not easy to identify for purposes of spotting patterns orcharacteristic damage to the scaffold, in contrast to damage that wouldbe caused if the crimper blades were not properly calibrated, bearingsneeded replacement, scaffold was not properly placed at a centralportion of the crimper, etc.

As is generally known in the art, the nature of deformation of anarticle through externally applied forces may, in some situations, beinferred from the reaction forces applied by the article against thebody, through which the external force is applied. For example, if thebody applying the force to the article is programmed to enforce adisplacement at a prescribed rate, monitoring the changes in the forceneeded to maintain the enforced displacement can give clues as to howthe body is being deformed. In the case of a scaffold, an operator canset the rate for crimping and monitor the applied force. However, theknown methods for instrumentation are not capable of providing the levelof accuracy needed to infer how individual struts are being deformed bycrimper jaws. The operator, therefore, has virtually no knowledge abouthow the scaffold's struts are being deformed within the crimper. Theonly knowledge that the operator has about how the scaffold might havebeen deformed when in the crimper occurs is after the scaffold iswithdrawn from the crimper and visually inspected. At this pointirreparable damage has occurred and the scaffold and catheter must bediscarded.

The art has dealt rather extensively with improving crimping processesfor metal stents. However, the assumptions made about aballoon-expandable metal stents when improving a crimping process, orproblem-solving, have ignored, or underestimated significant differencesbetween a polymer scaffold and a metal stent. First, irregulardeformations of metal struts, while not desirable, seldom occur. Andwhen they do occur, irregular deformations of metal struts are oftenacceptable. The same is not true of a polymer scaffold due to theinferior stress-strain characteristics of the polymer material. Second,polymer scaffolds are more susceptible to irregular deformations thanmetal stents due to the reduced space between polymer struts vs. metalstruts (polymer struts are normally thicker and wider than metal struts,so that the polymer struts have about the same radially stiffnessproperties). The existing art pertaining to crimpers fails to adequatelyaccount for these differences.

FIGS. 2-4 are illustrations referred to show relationships betweencrimper blades and a scaffold during a crimping process when using aconventional crimping process. For simplicity, the crimper head is drawnas having only 8 blades each spanning 45 degrees. A more typicalarrangement has 12 blades spanning 30 degrees each.

FIGS. 2 and 3 are cross-sectional views of a crimper head and scaffoldwithin the aperture of the crimper head showing the orientation of theblades relative to the scaffold when the aperture forms a first diameterand second, smaller diameter, respectively. The scaffold body 10 isdisposed between the blades 22. The scaffold 10 is supported on thecollapsed balloon 200 of the catheter when it is placed in the crimperhead. Then, as the blade edges engage the scaffold the scaffold islifted off the balloon as shown. This setup is typical setup for acrimping sequence for a metal stent, but with the metal stent replacedby a polymer scaffold.

In FIG. 2 the blade edges are directed away from the scaffold surface sothat only the more flat surface of the blade 22 abuts the scaffoldsurface when the aperture is at the first diameter. This means that theloading on the scaffold surface is distributed out more over the bodybecause there is more surface-to-surface contact, thereby more likelyavoiding blade indentations from forming in the scaffold. However, whenthe scaffold is at this large diameter problems can still occur. Sincethe scaffold diameter is much larger than the balloon profile, anyslight misalignment, either with regards to the scaffold relative tocatheter support, or the crimping blades not closing on the scaffolduniformly, can produce twisting or irregular bending as the scaffold isdisplaced off-center relative to the central crimping axis. The problemwas found to reside with the lack of an interior, stabilizing supportfor the scaffold when it has the large initial diameter, e.g., 2.5 to 3times the crimped diameter size.

In FIG. 3 the edges of the blades are directed more towards the surfaceof the scaffold because the blades are forming an iris that is smallerin size than in FIG. 2. In this arrangement, the relatively sharp edgesof the wedge-like blades 22 are bearing down on the scaffold surface. Itis in the situation that indentations, cuts and gouges causingunacceptable structural damage can occur. Moreover, if any twisting,irregular bending, or strut overlapping has begun to occur when thescaffold is at its larger diameter (FIG. 2), further twisting, bendingand overlapping of struts can occur. In particular, any bending ortwisting that has been initiated at the larger diameter is believed tobecome more pronounced or encouraged as the spacing between the strutsis reduced and struts begin to abut each other. As one strut twists outof alignment, it contacts an adjacent strut which can cause the adjacentstrut to be thrown out of alignment or become folded over the bentstrut. As mentioned above, this problem of abutting struts is notusually present for a metal stent because either a metal stent strut isthinner or the diameter reduction required for crimping is less than fora polymer scaffold. In either case, there is more spacing betweenstruts. Therefore, there is less chance that a misaligned strut willcause greater misalignment at smaller diameters because there is moreavailable clearance between struts as the stent is reduced in diameter.

FIGS. 4A and 4B are close up views of FIGS. 2 and 3, respectively,showing the relative position of the blade edge 22 b and surface 22 aaway from the edge 22 b relative to the surface of the scaffold 10. FIG.4B depicts a deformation, tearing or gouging being formed in thescaffold structure as a result of the edge 22 b bearing down upon thescaffold. This situation is believed to occur when an edge of the bladecatches the edge of a scaffold strut, such as a strut that waspreviously deformed or twisted, even slightly, out of position so thatthe blade edge 22 b catches it. As depicted, strut or crown 10′ has beentwisted and through abutment with strut 10″ results in the struts orcrowns overlapping each other. Once this interaction between strutsbegins and the diameter is reduced further, the problem can become worseand worse until the scaffold becomes unusable. As a result, both thescaffold and catheter supporting the scaffold is discarded.

As will be appreciated, it is very difficult to know the exact mechanismof action, or sequence of events leading to the situation depicted inFIG. 4B or that shown in FIGS. 11-12 and 14, for two reasons. First, onecannot generally visually inspect the interaction between blades and ascaffold. One can only inspect what the crimped shape looks like afterthe scaffold is removed from the crimper. Second, a polymer scaffold istypically transparent, or at best semi-opaque. When crimped to atransparent balloon, it can therefore be extremely difficult todetermine the exact nature or extent of cuts or indentations across thebody of the scaffold in order to spot a pattern or characteristicdamage.

It was also discovered that polymer scaffolds are susceptible to damageif they have a slight misalignment with the blades of the crimpingassembly of crimper. A “slight” misalignment means a misalignment thatthe art has tolerated in the past and assumed were present but notcapable of significantly effecting how a metal stent would be deformedby the crimper as compared to the same stent when perfectly aligned withand coming into contact with the blades of the crimper. Suchmisalignment tolerance is understood by reference to informationavailable from a manufacturer of a commercially available crimpingdevice. One type of misalignment of crimper blades believed to causeunacceptable damage to polymer scaffold would be when one blade is notmaintained flush with an adjacent blade, such that when the irisdiameter is reduced a sharp leading edge is exposed. This sharp edge canthen tear into, or cut across a polymer strut, e.g., resulting in thedamage shown in FIGS. 12A-12B.

FIGS. 11-12 and 14 are photographs showing damage to polymer scaffoldusing a known crimping process for metal stents. FIGS. 13 and 15 show,however, a marked improvement in crimping when processes according tothe disclosure are used in place of the known crimping process. FIGS.11-13 are photographs for the scaffold depicted in FIG. 5. FIGS. 14-15are photographs for the scaffold depicted in FIG. 10.

FIGS. 11A-11B show two types of defects in a crimped scaffold. FIG. 11Ashows that the linking element connected to the crown is bent. This bent“Y” section of the scaffold caused the crimped scaffold to be rejected.The region where the bending has occurred is a known high stress areawhere fracture is more likely to occur in the deployed scaffold. Forthis reason, the scaffold is rejected. Similarly, FIG. 11B shows adefect that will cause this scaffold to be rejected as well. The defecthere is, not only, a bent “Y” section, but also a pinching of the crown.As can be seen in this photograph, an upper crown is pressing againstthe lower crown forming the “Y”. The result is the struts are squeezedtogether beyond their designed bending ranges. Balloon expansion fromthis abnormal shape, therefore, raises concerns over loss in stiffnessor fracture toughness for the scaffold, in this area or areas nearby.FIGS. 12A and 12B are scanning electron microscope images showingabrasions, cuts, gouges and indentations in the scaffold due to theknife-like edge of the crimper blade bearing down on the scaffold. Thisdamage to the scaffold is likewise unacceptable and results in thescaffold being rejected. When using a conventional crimper head, damagelike that shown in both FIGS. 11 and 12 are likely present in thecrimped scaffold. Indeed, before the invention there was a near 0% yieldfor scaffold that were devoid of the damages depicted in FIGS. 11-12.

FIGS. 14A-14C depicts damage observed for the scaffold described in FIG.10. As can be readily seen, in FIG. 14A the scaffold exhibits severeirregularity, e.g., bent and even flipped struts, overlapping struts andpinched struts. FIG. 14B, while not showing the severity in bending andtwisting as in the case of FIG. 14A, nonetheless is also an unacceptablecrimping for a scaffold since Y or W elements at one circumferentialstation are being crimped at a different rate from anothercircumferential station. It is believed that if this scaffold werecrimped further, overlapping, flipping or similar undesired deformationof the scaffold will occur. FIG. 14C shows a close-up of overlappingstruts. This scaffold, as was the case of the scaffold depicted in FIGS.14A-14B, was rejected. The overlapping struts in this section can resultin improper deployment, as well as significant loss in stiffness andfracture toughness.

The inventors discovered, unexpectedly, that if “slight” misalignmentswere removed, or substantially removed, when crimping a polymerscaffold, there can be significant reductions in the irregulardeformations of scaffold struts that are sufficient to cause irreparabledamage to a polymer scaffold, e.g., a PLLA scaffold. A misalignmentrefers to either the scaffold bore axis not aligning with the crimpercentral axis or the scaffold not aligning properly with the blades ofthe crimping device axis as the iris is being closed onto the scaffold.One may view the two as global verses local misalignment. Betteralignment of the scaffold body and better support of the scaffoldrelative to the moving blades within the crimper was found to yieldimproved results, particularly when the scaffold requires a significantdiameter reduction and a high retention force.

Again, it should be mentioned that a polymer scaffold, and in particulara misaligned polymer scaffold is more susceptible to damage within acrimper than a corresponding metal stent. A polymer scaffold that has a“slight” misalignment within the crimper has a far greater chance ofbecoming damaged than a metal stent. Of course, the need to avoidtwisting, bending or indentations in struts of metal stents when in acrimper is known. However, unlike metal stents, which are far moretolerant to local irregular or non-uniform forces acting on strutsthrough blade edges, a polymer scaffold surface has a much lowerhardness than a metal stent surface. Therefore, the polymer scaffold ismore susceptible to local damage by the crimper blades. Moreover, due tothe proximity of struts to each other (as required since thicker andwider struts are needed to provide equivalent stiffness to a metalstent), there is a greater chance of abutting struts which leads to outof plane twisting and overlapping scaffold structure in the crimpedstate. The affect of irregular or non-uniform crimping forces on apolymer scaffold are therefore more severe than in the case of a metalstent. The differences are most clearly evident in the instances ofcracking and/or fracture in deployed polymer scaffolds that exhibitirregular twisting or bending and indentions.

Crimping a polymer scaffold in the manner illustrated in FIGS. 2 and 3was found to be inappropriate for these reasons. Scaffolds crimped inthis manner are often damaged and of no use. In one example using acrimping device and crimping process for metal stents the yield was near0%. When aspects of the invention were employed, the yield was increasedto 80%.

A crimping assembly according to the disclosure may adopt an iris-typeactuating mechanism alluded to above, an example of which is describedin U.S. Pat. No. 7,389,670, which disclosure, including all drawings, isfully incorporated herein for all purposes. FIG. 9 is a perspective viewof such a crimping assembly 20 incorporating a crimping mechanism thatincludes a collection of blades, e.g., twelve 30 degree blades, thatarticulate circumferentially and radially where each blade isarticulated radially and circumferentially in unison with the otherblades by a two axis linking mechanism (combined radial andcircumferential motion for each blade). The assembly thus includes aplurality of blades arranged to form an aperture 21 with variablediameter. The crimping assembly 20 includes a base 48, supports 50′ and50″ and an arm 65 for causing the blades to move inwards or outwards forreducing or enlarging, respectively, the aperture 21 formed by the iris.The central axis for the crimping blades, or crimping axis for theassembly is drawn as axis 31 in FIG. 9.

The problems previously described above with existing crimper assembliesfor polymer scaffolds were addressed by methods including (1) modifyingthe contacting surfaces of the crimper blades with the scaffold, (2)supporting the scaffold from the inside using balloon pressure.Embodiments of these aspects of the disclosure are provided.

In one embodiment a polymer scaffold is crimped using a crimper assemblythat supplies sheets of polymer film between the scaffold and crimperblades. An example of this type of crimping assembly is illustrated inFIGS. 1A and 1B. It was found that the polymer sheet also helped toreduce indentations in scaffold surfaces since the polymer sheetseffectively made the blade surfaces more compliant. However, disposing apolymer material between the blades and scaffold surface was not enoughto reduce damage to the polymer scaffold. An unacceptably high number ofdamaged scaffolds still resulted when the crimped scaffold was removedfrom the crimper of FIGS. 1A-1B.

It was believed that the sheets of material, while reducing indentationsdue to the effective reduction in compliance of the blades, also imposedtwisting forces on the scaffold, which promoted irregular bending ortwisting in the struts of the scaffold when the blades bore down on thescaffold surface. While not wishing to be tied to any particular theory,it is thought that by introducing sheets of material between thescaffold and blades, the tension on the sheets, combined with themovement of the blades relative to the sheets may have lead to theundesirable consequence of increasing the twisting of the unsupportedscaffold body as the diameter was reduced, which exacerbated, in somerespects, the irregular crimping observed. In another sense, it wasbelieved that the irregular deformations of struts caused by individualblades could not be reduced enough to increase the yield of usablescaffold-catheter assemblies when only a more compliant surface wasintroduced by way of the polymer sheets. In an attempt to produce moreuniform crimping and thus more acceptable yields, an interior supportfor the scaffold was introduced during an initial diameter reduction,e.g., reducing the scaffold diameter to about ½ its starting diameter.An inflated balloon was used to support the scaffold. An inflatedballoon was also employed when the scaffold was reduced down to itsfinal crimped diameter. It is not known, for certain, whether theimproved yield of scaffold-catheter assemblies was due solely to, ormostly to the use of a balloon support during the initial diameterreduction or the combination of inflated balloons during severalincremental crimping steps. As explained above, the precise cause andeffect resulting in damaged scaffold structure is not easilydeterminable due to complex nature of the inelastic deformation of thepolymer material and inability to closer inspect each phase of thecrimping sequence. Nevertheless, testing reveals that when balloonpressure provides support for the scaffold, the yield ofscaffold-catheter assemblies improves dramatically.

More local support for individual struts when the scaffold nears itsfinal crimped diameter is believed to add some measure of support forstruts predisposed to twist or overlap with adjacent struts (a strutpredisposed to twist or overlap with other struts refers to a strut thatwas previously slightly bent or twisted out of plane when the scaffoldwas at a larger diameter. As discussed earlier, due to the proximity ofstruts for a polymer scaffold, as opposed to a metal stent, there istherefore a greater likelihood of bending, twisting or overlap as strutsabut each other). In essence, balloon pressure is believed to provide abeneficial reacting pressure upon the luminal side of the strut, whichcan serve to limit a strut's potential to overlap or twist irregularlywhen a blade edge imparts a higher degree of force to a strut than theblade applied during an earlier crimping step.

Balloon pressure helps to stabilize the scaffold during the initialphases of the crimping sequence. In one example, the scaffold is reducedfrom an over-deployed or deployed diameter to a diameter that about 2.5to 3 times smaller in size. When at the deployed or over-deployeddiameter, there is little stabilizing support for the scaffold since itsdiameter is much larger than the deflated balloon catheter upon whichthe scaffold sits. As such, any initial non-uniform applied crimpingforce, or misalignment, e.g., due to a residual static charge on thepolymer surface, can initiate irregular bending that becomes morepronounced when the scaffold diameter is reduced further. Frictionbetween the blades and the scaffold surface, or residual static chargeor static charge buildup induced by sliding polymer surfaces are alsosuspect causes of this irregular deformation of the scaffold. When theballoon was inflated to support the scaffold from the interior, it wasdiscovered that the irregular bending and twisting of struts werereduced substantially. The scaffold was more able to maintain a properorientation with respective to the crimper axis. The uniform pressureapplied by the balloon tended to balance-out any non-uniformity in theapplied crimping force.

Additional crimp refinements were employed by the inventors in an effortto improve scaffold-catheter assembly yield. First, polymer surfaceswithin the crimper head, whether in the form of polymer sheets orcoatings disposed on the blades (as discussed in greater detail, below),are deionized prior to crimping to avoid static charge buildup. Second,scaffold temperature is raised to near the glass transition temperatureof the polymer to reduce instances of crack formation during crimping(as well as to increase balloon retention), but without affecting thedeployed structure's strength and stiffness profile. These additionalimprovements to polymer scaffold crimping processes are discussed inmore detail in U.S. application Ser. No. 12/776,317 and U.S. applicationSer. No. 12/772,116. These applications share a common inventor andassignee with the present application.

Examples of crimping sequences/protocols for reducing damage to apolymer scaffold will now be discussed. In these examples, the scaffoldwas formed from a radially expanded tube of PLLA. The scaffold had astrut pattern as shown in FIG. 5, 6 and FIG. 10. An iris crimper havingsimilar actuating characteristics to the crimping assembly described inconnection with FIG. 9 was used to pre-crimp and final crimp thescaffold to the balloon.

A crimping process for a polymer scaffold having the scaffold pattern ofstructural rings, struts and linking elements shown in FIG. 5, 6 or 10may proceed as follows. In preparation for the initial diameterreduction, the scaffold is placed on a balloon and deionized, thepolymer sheets disposed within the crimping aperture 21 are deionized,and the scaffold is placed with the crimper 20. The temperature of thecrimper blades are raised to, or to about 48° C. and allowed tostabilize at that temperature. The radiant and convective heat from theblades is relied on to raise the temperature of the scaffold when thescaffold is in the crimper. Hot air may also be introduced to raise thescaffold temperature.

Unlike a metal stent, a polymer scaffold of the type illustrated in FIG.5, 6 or 10 requires an initial diameter reduction or pre-crimp, are-alignment or alignment check with balloon markers, followed by afinal crimp procedure due to the initial large diameter of the polymerscaffold, which large diameter may correspond to a deployed or overdeployed diameter for the scaffold. A polymer scaffold is formed in thisway so that it can possess the polymer chain alignment most optimal forproviding high stiffness and low recoil in the deployed state. Thisconfiguration, however, also makes the crimping process more challengingbecause there is large diameter reduction needed (metal stents, incontrast, due not require this form of assembly due to the difference inmaterial properties. Metal stents may be fabricated at a reduceddiameter, which makes it far easier to crimp the metal stent to aballoon since the starting diameter is closer to the crimped diameter).In one embodiment, a scaffold is reduced from a starting diameter ofabout 0.136 in to a crimped diameter of about 0.052 in. In anotherembodiment, a scaffold is reduced form a starting diameter of about 9 mmto a crimped diameter of between about 2 and 3 mm.

In the embodiments, an anti-static filtered air gun is used to deionizethe scaffold before and/or during pre-crimping. Before pre-crimp, theanti-static air gun is passed over the scaffold front to back to removestatic charges on the scaffold. In one case, the anti-static filteredair gun is applied for 10 seconds to 1 minute along the scaffold. Inanother embodiment, the air gun deionizes the scaffold duringpre-crimping. The anti-static filtered air gun is applied for 10 secondsto 1 minute along the scaffold.

EXAMPLES

The crimping sequence for a 3.0×18 mm PLLA scaffold having the patternillustrated in FIG. 5 is illustrated as an example. The initialpre-crimp moves the blades forming the iris from a starting diameter of0.136 in to a diameter of 0.083 in where it remains for a 30 seconddwell. This is stage 1. During this stage balloon pressure may beapplied to stabilize the scaffold, in an amount of about 2 to 15 psi.The scaffold and balloon upon which the scaffold rests is then removedfrom the crimper and the alignment with balloon markers verified. Thediameter reduction to 0.082 in loosely secures the scaffold to theballoon so that it can hold its place but is still capable of beingadjusted relative to balloon markers. The scaffold is returned to thecrimper.

Stage 2 of the crimping sequence moves the blades forming the iris fromto a 0.068 in and is held for 15 seconds. During this stage, the balloonis inflated to about 17 to 100 psi. After this stage is complete, theballoon is deflated and the iris opened to allow the catheter to beremoved. The scaffold receives a final alignment to the balloon markers.The scaffold and balloon are placed back into the crimper. Stage 3reduces the diameter to 0.070 in with a 10 second dwell. During thisstage 3, the balloon is inflated to about 17 to 100 psi. Once complete,the machine moves to Stage 4, where the balloon pressure is reduced tolower than about 15 psi and iris reduced to 0.047 in and held for afinal 200 second dwell. When this fourth and final stage is complete,the iris is opened and the catheter and scaffold removed. The scaffoldis retained on the balloon and immediately placed into a sheath minimizerecoil in the polymer scaffold.

A balloon pressure during diameter reduction may be selected to providesupport for the scaffold without imposing excessive stresses on theballoon material. Alternatively, a compliant and expendable supportballoon held at a constant pressure may be used during the initialdiameter reduction, as in the case of the scaffold of FIG. 10. In someembodiments, the balloon pressure may be adjusted by a controlledrelease of gas pressure as the scaffold diameter is decreased. In otherembodiments, the balloon pressure may be increased after an incrementaldiameter reduction is made, during a dwell period. By increasing balloonpressure immediately after an incremental crimp, any irregulardeformations can be adjusted by supporting balloon pressure, whichprovides a uniform pressure to the inner surfaces of the scaffold tocompensate for any tendency for a strut to irregular deformation. Forexample, a strut that was deformed inwardly can be pushed back intoposition when the balloon is inflated.

In another embodiment a scaffold reduced in diameter from about 9 mm toabout 2-3 mm has a 120 mm length. For this scaffold the crimpingsequence may proceed as follows using a crimping station such as acrimping station described in U.S. application Ser. No. 12/831,878.

In a first and second example, a crimp process for the scaffold depictedin FIG. 9 is crimped at a temperature of about 48 degrees Celsius forPLLA scaffold material. For PLGA the temperature may be lower. Thescaffold temperature is raised via convection and radiation from theheated crimper blades.

The 9 mm ID scaffold is placed on a 9-10 mm support balloon. Thisballoon is inflated through a sidearm of the balloon with 40-70 psi airto create a balloon OD of 8 mm. Keep the support balloon pressurized.Place this scaffold-balloon assembly on the loading carriage. Push thecarriage forward until the assembly is in the center of the crimp head.

First example of a crimp process following the scaffold-balloon placedin crimp head:

Stage 1—crimp head closes to 0.314″ at a speed of 0.5 inches per second(in/s) then immediately go to Stage 2.

Stage 2—crimp head closes to 0.300″ at a speed of 0.005 in/s and dwellsfor 30 seconds.

Stage 3—crimp head closes to 0.270″ at a speed of 0.005 in/s and dwellsfor 30 seconds. Turn stopcock to release pressure from the inflatedsupport balloon catheter.

Stage 4—crimp head closes to 0.240″ at a speed of 0.005 in/s and dwellsfor 30 seconds.

Stage 5—crimp head closes to 0.200″ at a speed of 0.005 in/s and dwellsfor 30 seconds.

Stage 6—crimp head closes to 0.160″ at a speed of 0.005 in/s and dwellsfor 30 seconds. Activate pressurization mode of crimping station toinflate the support balloon with 50 psi to align any misaligned strutsbetween Stage 3 and Stage 5. After dwelling for 30 seconds the crimphead opens, remove the scaffold/support balloon from the crimp head.Remove partially crimped scaffold and place it on the balloon of theballoon catheter (“FG balloon catheter”). Insert this assembly back intothe center of the crimp head. Reactivate the crimper.

Stage 7—crimp head closes to 0.160″ at a speed of 0.25 in/s and dwellsfor 30 seconds.

Stage 8—crimp head closes to 0.130″ at a speed of 0.005 in/s and dwellsfor 50 seconds. Activate pressurization mode to inflate the FG ballooncatheter 50 psi to create pillowing effect to improve scaffold retentionand dwell for 50 seconds. Deactivate pressurization mode after 50seconds have elapsed.

Stage 9—crimp head closes to 0.074″ at a speed of 0.005 in/s and dwellsfor 150 seconds.

Remove finished scaffold-catheter assembly from crimp head andimmediately place restraining sheath over scaffold to limit recoil.

Second example of a crimp process following the scaffold-balloon placedin crimp head.

Stage 1—crimp head closes to 0.314″ at a speed of 0.5 inches per second(in/s) then immediately go to Stage 2.

Stage 2—crimp head closes to 0.160″ at a speed of 0.005 in/s and dwellsfor 30 seconds. During this stage a relief valve releases pressure fromthe pressurized support balloon catheter to prevent balloon rupture.After dwelling for 30 seconds the crimp head opens, remove thescaffold/support balloon from the crimp head. Remove partially crimpedscaffold and place it on a FG balloon catheter. Insert the subassemblyback into the center of the crimp head. Reactivate crimper.

Stage 3—crimp head closes to 0.130″ at a speed of 0.005 in/s and dwellsfor 50 seconds. Activate pressurization mode to inflate the FG ballooncatheter to 50 psi to create pillowing effect to improve scaffoldretention and dwell for 50 seconds. Deactivate pressurization mode after50 seconds have elapsed.

Stage 4—crimp head closes to 0.074″ at a speed of 0.005 in/s and dwellsfor 150 seconds.

Remove finished scaffold-catheter assembly from crimp head andimmediately place restraining sheath over the scaffold to limit recoil.

In yet another alternative to these crimping processes, in a thirdexample the scaffold is rotated about its axis while supported on thesupport or temporary balloon between intermediate crimping stages. Thus,after an initial crimp, the scaffold and support balloon are removedfrom the crimper head and the scaffold is rotated, e.g., about 45degrees about its axis, then a second crimp is performed. The same stepmay be performed several times until the diameter is reached in whichthe temporary balloon is replaced by the balloon catheter. In anotherexample, the rotation may be less than 30 degrees, or the angleextending between adjacent “Y” shape elements. The angle of rotation mayalso be the ½ angle between Y-shaped elements to compensate for anon-uniform crimping such as that depicted in FIG. 14B.

In other embodiments a polymer coating is applied to edges of blades,rather than using tensioned polymer sheets as in FIG. 1B. The effect isto reduce the hardness of the blade surfaces that contacts the polymerscaffold, or to cause the crimper blade loading of the scaffold strutsto by more widely distributed over the surface. In another sense, theobjective is to make the blade edge softer so that its hardness iscloser to that of the relatively soft polymer surface. In doing so, theblade forces (especially at or near the blade edge) will be distributedover a greater portion of the surface of the scaffold (since the surfaceis made more soft) which should reduce indentations on the scaffoldsurface, especially when the iris diameter moves to the final crimpdiameter (FIG. 3). Hardness is meant to refer to the resistance of asurface to permanent shape change when a force is applied. For presentpurposes, hardness refers to indentation hardness, or the ability toresist a permanent indentation from forming. Since it is not desired tochange properties of the polymer scaffold so as to affect its hardness,the hardness of the blade is changed, i.e., it is made softer, byapplying a polymer coating of suitable hardness to the blade edge.

On the one hand, one may wish to match the hardness of the coated bladeto the scaffold surface, which is intended to mean the “effective”hardness of the blade, i.e., the hardness of the coated surface thatcomes into contact with the scaffold surface. This arrangement wouldperhaps be most ideal from the standpoint of avoiding indentations inthe scaffold while ensuring the blades are capable of deforming thescaffold struts in the intended manner. On the other hand, reducingblade hardness to this degree would require more frequent maintenance ofthe blades as the coated blade edge would become deformed or removedfrom the blades relatively often (depending on the material used)following a production crimping run. Reducing blade hardness so that itis about at the hardness of the scaffold may also not be desired whencrimping at elevated temperatures.

For example, to reduce the blade hardness to the hardness of thescaffold there may be a relatively thick coating requirement, or apolymer material may be needed that has a relatively low heat transfercoefficient. In either case, the polymer coating used to match hardnessmay make it difficult to effectively or efficiently conduct heat fromthe blades to the scaffold in those cases where the scaffold is heatedby heat conducted and radiated from the metal blades.

The polymer coating may be polyurethane or any other relatively elasticpolymer material. The coating thickness applied to blades may range fromabout 100 to 150 microns, depending on the material used. The thicknessof the coating may be selected to make the edge of the crimper bladesmore soft but without causing thermal insulation problems. For example,a polymer coating thickness may be maintained at a constant thickness,or having a tapered thickness so that damages caused by sharp edges arereduced yet the scaffold can be efficiently heated to a desired crimpingtemperature by way of blade radiation/conduction.

According to one embodiment, a scaffold inserted within a crimper headexposed to crimper blades will obtain a temperature at about the glasstransition temperature of the polymer, and more preferably between 5 or10 degrees below the glass transition temperature without additionalheating sources being required for a tapered polymer thickness over theblade edge contact length, or less than this length, with a maximumthickness being at or near the sharp tip being between about 100 and 150microns. As alluded to above, if the coating is too thick or disposedover much of the tip of the blade, then heat convection from the bladesto the scaffold may become impaired which makes scaffold heating throughthe crimp blades infeasible, or impractical for batch or productioncrimping. In addition, or alternatively, the hardness of the edgemodified by the coating to reduce indentations from forming in thescaffold may also make the blade more susceptible to deformation (sincethe surface is softened), which may necessitate frequent maintenance ofthe polymer coated blades.

The polymer coating may further, or in addition to, be evenly appliedover the edge of the blade, or applied non-uniformly according to theshape or orientation of the blade relative to scaffold surface at thefinal crimp diameter. The coating may be applied over both the edge andthe surface proximal the edge that contacts the scaffold when the irisis at a larger diameter. Or the coating may be limited to the edge toonly compensate for damage believed to occur primarily when the irisapproaches the final crimped diameter. The thickness and/or distributionof coating over the blade may be selected based on a need to maintain aminimum rate of heat convection across the contacting surface orradiated heat from the exposed metal surface to the scaffold surface, orbased on the particular blade design and/or where in the crimpingsequence damage is believe to most likely occur, e.g., at the finalcrimp or earlier in the crimping sequence.

In other embodiments the blade edge may be configured to receive aremovable polymer insert, or edge to facilitate more efficient upkeepand reduce downtimes over embodiments that use a polymer coating. Anexample of such an insert is described in U.S. Pat. No. 7,389,670.Inserts, as opposed to an applied coating, however, can only be made sosmall and/or thin to enable the insert to be easily secured to, andremoved from the blade edge. As such, a blade that uses a polymerinsert, e.g., as disclosed in U.S. Pat. No. 7,389,670 may introducethermal insulation problems between the blades and the scaffold. Assuch, it may not be desirable to use an insert when the metal surfacesof the blades are needed to conduct heat to the polymer scaffold.

Embodiments are illustrated in FIGS. 7A and 7B. In FIG. 7A, the coating50 on blade 22 has a first thickness t2 at the leading edge 22 b whichtapers to a third, reduce thickness t3 away from the leading edge atsurface 22 a. In FIG. 7B, a generally constant thickness “t1” of coating50 is applied at the edge 22 b and over the surface 22 a, which contactsthe strut surface before the leading edge 22 b during the crimpingprocess.

FIG. 7C depicts a blade 22′ formed to receive a polymer insert 51. Theinsert has about the same thickness t4 over the distance the scaffoldmakes contact with the blade surface and is shaped to approximately formthe edge and surface dimensions of the blades of FIGS. 7A-7B. As can beappreciated by comparing the relative thicknesses, when a replaceableinsert is used (FIG. 7C) the insert can thermally insulate the metalblade from the scaffold, which is not desired when blade heat is used toheat the scaffold as in the preferred embodiments. Thus, for embodimentsof invention in which a polymer scaffold is heated by the blades, acoating is used which can have a thickness that does not adverselyimpact heat convection or radiation from the blades to the scaffold.

In other embodiments the blade edge 22 b may be reshaped to provide amore blunted or rounded edge to reduce force concentrations on thescaffold surface when the iris approaches the final crimped diameter.The objective sought for such a blade tip may be two-fold. First, byproviding a more rounded or blunted edge or tip (a rounded edge beingone embodiment of a blunted edge) the surface-to-surface contact areabetween the blade and scaffold can be made more constant throughout thecrimping steps. This has the effect of reducing damaging forceconcentrations produced by a narrow blade edge, which forceconcentrations result from a narrow contact area over which the bladeapplies the crimping force near the final crimping diameter. As such, byincreasing the surface area over which the blade acts on the scaffoldindentions can be reduced. Second, by providing a blunted edge free fromrelatively dramatic changes in the surface over which the blade acts onthe scaffold, especially when blades become misaligned (e.g., as aresult of crimper bearings beginning to wear), any previouslyirregularly deformed scaffold struts caused by prior crimping steps in acrimping sequence will have less tendency of being caught, grabbed, orpushed outwardly or inwardly by a blade edge. It is believed thatsignificant damage may occur during the final crimping steps from thistype of interaction between a blade edge and a previously deformedstrut.

An example of these embodiments is illustrated in FIGS. 8A and 8B (thewidth of the blade 24 is exaggerated in this view, as compared to FIGS.7A-7C, for ease of illustration). These drawings show a blade 24 edgethat has been modified to make it blunter. In these examples the edge ismade rounded. The blade edge 24 b has a curvature defined by a radius ofcurvature R with the center of the circle being offset by a distance “d”from a bisecting line 37 of the converging surfaces 38, 39 that definethe width of a wedge that terminates at a reference point “p”. Forexample, if there are twelve crimper blades that cooperate to form theiris, then each blade defines a wedge spanning 30 degrees. Therefore,the angle φ in FIG. 8B is 15 degrees. The bisecting line 37 maycorrespond to the line of action of the blade 24 when it moves inward bythe mechanism of the crimping assembly (FIG. 9). The blade edge may beasymmetric with respect to the bisecting line 37.

As the blade 24 rotates counterclockwise in FIGS. 8A-8B (correspondingto a smaller iris) the blunt edge 24 b of the blade 24 more or lessmaintains the same amount of surface-to-surface contact with thescaffold surface as the preceding surface 24 a did when the scaffold hada larger diameter. By maintaining the same surface contact per blade,force concentrations on the scaffold surface resulting in indentationsshould be reduced. The surface 24 a, which contacts the scaffold surfaceat larger radii of the iris, is sloped to provide a gradual change incurvature leading to the radius of curvature at the edge 24 b. Abruptchanges in the surface contour of the blade 24 are not desired so thatforce concentrations can be avoided when the blade bears down on thescaffold.

FIG. 8B shows a polymer coating 52 applied to the blunted edge, whichmay be beneficial as a measure to form a more circular iris at thesmaller diameters, or to reduce the blade hardness. Despite beingblunted, the blade of FIG. 8A may still damage the scaffold when itabuts the relatively soft surface of the scaffold. As can be appreciatedby comparing the outer surface of the coated and blunted edge embodimentof FIG. 8B with the other drawings of the blade edge, the blade 24 hasabout the same surface contours as blade 22 without the coating applied.In cases where the polymer coating is formed to mimic the sharp edge ofthe convention blade tip, the polymer used for coating 52 may be lesselastic or harder since it is formed into a narrow edge. In this way,the polymer edge will have more ability to retain its shape afterseveral scaffold are crimped using a blade configured in this manner.

There are also beneficial effects of forming a blunted, asymmetric edgelike that shown in FIGS. 8A-8B relating to avoiding twisting, bending oroverlapping struts. When a relatively pointed blade edge, e.g., asdepicted in U.S. Pat. No. 7,389,670, bears down on a strut, it cancontact or catch a side surface of a strut to cause the strut to bendoutwardly or inwardly, especially when the strut was previously bentoutwardly or inwardly when the scaffold was being crimped at a largerdiameter. By forming a more blunt edge, the edge will have more of atendency to slide over the edge as the iris diameter is decreased,rather than catching or grabbing the previously bent or twisted strut atits side surface.

It should be pointed out that crimping assemblies heretofore proposedfor metal stents have suggested the opposite approach to thatillustrated in FIGS. 7-8. Others have proposed making the blade tipsmore inwardly curved so that when the blades come together at the finaldiameter the iris will form a more circular shape. However, it isbelieved this approach would actually exacerbate the problems theinventors are attempting to solve for polymer scaffolds, because, ingeneral, it is difficult to maintain perfect alignment of the blades. Aninwardly curved tip for the blade can actually increase damage to apolymer scaffold since an exposed leading edge is then brought moredirectly into the surface of the scaffold when the iris approaches thefinal crimped diameter, unless the blades are always maintained inperfect alignment. In this example it is seen that the art pertaining tometal stent crimper devices has proceeded in a direction opposite tothat proposed by the inventors. Indeed, based on the known art, itappears there has heretofore been little concern over whether a crimperblade might form indentations, cutting, tearing, or twisting of softermaterial used to form medical devices, such as a polymer scaffold, whichthe inventors have found are highly susceptible to damage resulting inloss of strength or improper deployment.

As noted above, according to the disclosure a scaffold has the scaffoldpattern described in U.S. application Ser. No. 12/447,758 (US2010/0004735). Other examples of scaffold patterns suitable for PLLA arefound in US 2008/0275537.

FIG. 5 shows a detailed view of an intermediate portion 216 of a strutpattern 200 depicted in US 2010/0004735. The intermediate portionincludes rings 212 with linear ring struts 230 and curved hinge elements232. The ring struts 230 are connected to each other by hinge elements232. The hinge elements 232 are adapted to flex, which allows the rings212 to move from a non-deformed configuration to a deformedconfiguration. Line B-B lies on a reference plane perpendicular to thecentral axis 224 depicted in US 2010/0004735. When the rings 212 are inthe non-deformed configuration, each ring strut 230 is oriented at anon-zero angle X relative to the reference plane. The non-zero angle Xis between 20 degrees and 30 degrees, and more narrowly at or about 25degrees. Also, the ring struts 230 are oriented at an interior angle Yrelative to each other prior to crimping. The interior angle Y isbetween 120 degrees and 130 degrees, and more narrowly at or about 125degrees. In combination with other factors such as radial expansion,having the interior angle be at least 120 degrees results in high hoopstrength when the scaffold is deployed. Having the interior angle beless than 180 degrees allows the scaffold to be crimped while minimizingdamage to the scaffold struts during crimping, and may also allow forexpansion of the scaffold to a deployed diameter that is greater thanits initial diameter prior to crimping. Link struts 234 connect therings 212. The link struts 234 are oriented parallel or substantiallyparallel to a bore axis of the scaffold. The ring struts 230, hingeelements 232, and link struts 234 define a plurality of W-shape closedcells 236. The boundary or perimeter of one W-shape closed cell 236 isdarkened in FIG. 5 for clarity. In FIG. 5, the W-shapes appear rotated90 degrees counterclockwise. Each of the W-shape closed cells 236 isimmediately surrounded by six other W-shape closed cells 236, meaningthat the perimeter of each W-shape closed cell 236 merges with a portionof the perimeter of six other W-shape closed cells 236. Each W-shapeclosed cell 236 abuts or touches six other W-shape closed cells 236.

Referring to FIG. 5, the perimeter of each W-shape closed cell 236includes eight of the ring struts 230, two of the link struts 234, andten of the hinge elements 232. Four of the eight ring struts form aproximal side of the cell perimeter and the other four ring struts forma distal side of the cell perimeter. The opposing ring struts on theproximal and distal sides are parallel or substantially parallel to eachother. Within each of the hinge elements 232 there is an intersectionpoint 238 toward which the ring struts 230 and link struts 234 converge.There is an intersection point 238 adjacent each end of the ring struts230 and link struts 234. Distances 240 between the intersection pointsadjacent the ends of rings struts 230 are the same or substantially thesame for each ring strut 230 in the intermediate portion 216 of thestrut pattern 200. The distances 242 are the same or substantially thesame for each link strut 234 in the intermediate portion 216. The ringstruts 230 have widths 237 that are uniform in dimension along theindividual lengthwise axis 213 of the ring strut. The ring strut widths234 are between 0.15 mm and 0.18 mm, and more narrowly at or about 0.165mm. The link struts 234 have widths 239 that are also uniform indimension along the individual lengthwise axis 213 of the link strut.The link strut widths 239 are between 0.11 mm and 0.14 mm, and morenarrowly at or about 0.127 mm. The ring struts 230 and link struts 234have the same or substantially the same thickness in the radialdirection, which is between 0.10 mm and 0.18 mm, and more narrowly at orabout 0.152 mm.

As shown in FIG. 5, the interior space of each W-shape closed cell 236has an axial dimension 244 parallel to line A-A and a circumferentialdimension 246 parallel to line B-B. The axial dimension 244 is constantor substantially constant with respect to circumferential positionwithin each W-shape closed cell 236 of the intermediate portion 216.That is, axial dimensions 244A adjacent the top and bottom ends of thecells 236 are the same or substantially the same as axial dimensions244B further away from the ends. The axial and circumferentialdimensions 244, 246 are the same among the W-shape closed cells 236 inthe intermediate portion 216.

It will be appreciated from FIG. 5 that the strut pattern for a scaffoldthat comprises linear ring struts 230 and linear link struts 234 formedfrom a radially expanded and axially extended polymer tube. The ringstruts 230 define a plurality of rings 212 capable of moving from anon-deformed configuration to a deformed configuration. Each ring has acenter point, and at least two of the center points define the scaffoldcentral axis. The link struts 234 are oriented parallel or substantiallyparallel to the scaffold central axis. The link struts 234 connect therings 212 together. The link struts 232 and the ring struts 230 definingW-shape closed cells 236. Each W-shaped cell 236 abuts other W-shapedcells. The ring struts 230 and hinge elements 232 on each ring 212define a series of crests and troughs that alternate with each other.Each crest on each ring 212 is connected by one of the link struts 234to another crest on an immediately adjacent ring, thereby forming anoffset “brick” arrangement of the W-shaped cells.

Referring to the scaffold pattern 300 depicted in FIG. 6, there arecylindrical rings 305 formed as connected diamond-like cells 310, eachring 305 being interconnected by horizontal linking elements 355. Thebending elements 320, 315, 335, 340, 345 form the cells 310. The cellsare connected at ends 360.

According to other embodiments a scaffold has a scaffold pattern asdepicted in FIG. 10. Examples of this scaffold pattern as described inU.S. patent application Ser. No. 12/561,971. The scaffold pattern 400includes a plurality of zig-zag like annular bands 306, 308. Eachannular band is connected by a horizontal linking element 304.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications can be made without departing from thisinvention in its broader aspects. Therefore, the appended claims are toencompass within their scope all such changes and modifications as fallwithin the true spirit and scope of this invention.

What is claimed is:
 1. A method for crimping a polymer scaffold to aballoon, comprising: providing a crimping assembly for crimping thescaffold from a first diameter to a second diameter, the crimpingassembly including a plurality of movable blades, each blade having ahardness, a first side and a second side converging to form a tip, thetips being arranged to collectively form an iris about a rotationalaxis, the iris defining a crimp aperture about which the movable bladesare disposed; disposing a polymer material between edges of the bladetips and a scaffold surface to reduce the hardness in the blade edges;supporting the scaffold while the scaffold is disposed within the irisincluding inflating a balloon within the scaffold to provide interiorsupport to the scaffold, whereby adjacent struts of the scaffold twistedirregularly by a crimper blade are supported by a balloon surface todeter one of the struts from overlapping or twisting irregularlyrelative to another of the struts; and displacing the plurality ofmovable blades from a first crimp head diameter to a second crimp headdiameter to reduce the diameter of the scaffold from the first diameterto the second diameter, respectively.
 2. The method of claim 1, whereinthe polymer material is a coating disposed on a leading edge of theblade.
 3. The method of claim 2, wherein the coating has a uniformthickness.
 4. The method of claim 2, wherein the coating has a coatingthickness that tapers away from the leading edge of a blade and along aside that contacts the scaffold surface when the scaffold has the firstdiameter.
 5. The method of claim 1, wherein the scaffold has ringstructures forming a tubular body having a distal end, a proximal end,and an intermediate segment between the distal and proximal ends, thering structures connected to each other by link struts oriented axially,the ring structures and link struts forming W-shape cells, and whereinthe inflating step includes maintaining a position of a first ringstructure relative to an adjacent, second ring structure by the balloonbeing inflated during the diameter reduction and/or after the diameterreduction.
 6. The method of claim 1, wherein the scaffold has zigzagannular bands connected by linking elements, and wherein the firstdiameter of the scaffold is about 2.5 to 3 times larger than the seconddiameter of the scaffold.
 7. The method of claim 1, wherein the scaffoldhas zigzag annular bands connected by linking elements, and wherein theballoon has a constant pressure during the diameter reduction.
 8. Themethod of claim 1, wherein a balloon pressure is decreased during thediameter reduction.
 9. The method of claim 1, wherein the scaffold hasannular bands connected by linking elements, and wherein the balloon isa disposable support balloon used to support the scaffold when thescaffold has the first diameter, wherein the scaffold is reduced indiameter by about 30-40% when reduced from the first diameter to thesecond diameter, and further including the step of reducing the scaffoldfrom the second diameter to a third diameter, including placing thescaffold on the scaffold delivery balloon before reducing the diameterto the third diameter.
 10. The method of claim 1, wherein the scaffoldhas an elevated temperature during the diameter reduction, the scaffoldis made from a polymer having a glass transition temperature, andwherein the elevated temperature is the polymer glass transitiontemperature or 5-10 degrees below the glass transition temperature. 11.The method of claim 10, wherein the polymer includes poly(L-lactide) andthe elevated temperature is between about 45 and 54 degrees Celsius. 12.A method for crimping a polymer scaffold to a balloon, comprising:providing a crimping assembly for crimping the scaffold from a firstdiameter to a second diameter, the crimping assembly including aplurality of movable blades, each blade having a hardness, a first sideand a second side converging to form a tip, the tips being arranged tocollectively form an iris about a rotational axis, the iris defining acrimp aperture about which the movable blades are disposed; disposing apolymer material between edges of the blade tips and a scaffold surface;supporting the scaffold while the scaffold is disposed within the iris;heating the scaffold to a glass transition temperature (Tg) of thepolymer of the scaffold, or 5 to 10 degrees below Tg; displacing theplurality of movable blades from a first crimp head diameter to a secondcrimp head diameter to reduce the diameter of the heated scaffold fromthe first diameter to the second diameter, respectively; and after thescaffold has been crimped to the balloon, placing the crimped scaffoldand balloon in a restraining sheath.
 13. The method of claim 12, whereinthe scaffold has a pattern of rings, wherein adjacent rings areconnected by links, and the rings and links form a plurality of cells,wherein a cell is formed by a first ring portion, a second ring portionand two links connecting the ring portions to each other, and both thefirst and second ring portions have 1 crown connected to a link thatconnects to an adjacent ring adjacent the respective first and secondring portion, and 4 crowns not connected to a link.
 14. The method ofclaim 12, wherein following the diameter reduction the scaffold isplaced on a catheter comprising the balloon.
 15. A method for crimping apolymer scaffold to a balloon, comprising: providing a crimping assemblyfor crimping the scaffold from a first diameter to a second diameter,the crimping assembly including a plurality of movable blades, eachblade having a hardness, a first side and a second side converging toform a tip, the tips being arranged to collectively form an iris about arotational axis, the iris defining a crimp aperture about which themovable blades are disposed; disposing a polymer material between edgesof the blade tips and a scaffold surface; supporting the scaffold whilethe scaffold is disposed within the iris; and displacing the pluralityof movable blades from a first crimp head diameter to a second crimphead diameter to reduce the diameter of the scaffold from the firstdiameter to the second diameter, respectively; wherein the scaffold hasa pattern of rings, wherein adjacent rings are connected by links, andthe rings and links form a plurality of cells, wherein a cell is formedby a first ring portion, a second ring portion and two links connectingthe ring portions to each other, and both the first and second ringportions have 1 crown connected to a link that connects to an adjacentring adjacent the respective first and second ring portion, and 4 crownsnot connected to a link; and after the scaffold has been crimped to theballoon, placing the crimped scaffold and balloon in a restrainingsheath.
 16. The method of claim 15, wherein the balloon is inflatedafter the diameter reduction and before the scaffold and balloon areplaced in the restraining sheath.
 17. The method of claim 16, whereinthe balloon is inflated to retain the scaffold on the balloon.
 18. Themethod of claim 15, wherein after the diameter reduction the scaffold isplaced on a catheter comprising the balloon.
 19. The method of claim 18,wherein the scaffold is placed on a support balloon before the diameterreduction.
 20. The method of claim 15, wherein the scaffold has anelevated temperature during diameter reduction, the scaffold is madefrom a polymer having a glass transition temperature, and wherein theelevated temperature is the polymer glass transition temperature or 5-10degrees below the glass transition temperature.